128 ENVIRONMENTAL SCIENCE
It is very simplified energy flow model of three trophic levels
Apparently the energy flows is greatly reduced at each successive trophic level from
producers to herbivores and then to carnivores. It is reflected that at each transfer of energy
from one level to another, major part of energy is lost as heat or other form. The energy flow
is reduced successively. We may consider it in either term as under:
(1) In terms of total flow (i.e. total energy input and total assimilation).
(2) In terms of secondary production and respiration components.
In this way of the 3,000 Kcal of total light, which falls upon the green plants,
approximately 50 per cent (1500 Kcal) is absorbed. Only 1 per cent (15 Kcal) of it is converted
at first trophic level. Thus net primary production comes to be at 15 Kcal. Secondary
productivity (P2 and P3 in the diagram) is about 10 percent at successive consumer trophic
levels in other words at the levels of herbivores and the carnivores. However, efficiency may
be sometimes higher as 20 per cent, at the carnivore level as shown (or P3=0.3 Kcal) in the
diagram.
It may be concluded from the above studies as under:
(1) There is a successive reduction in energy flow at successive trophic levels. Thus
shorter the food chain, greater would be the available food energy. The reason is
with an increase in the length of food chain, there is a corresponding more loss of
energy.
(2) With a reduction in energy flow (shown as ‘pipes’ in the diagram) at each successive
trophic level, there is also a corresponding decrease in standing crop or biomass
(shown as ‘boxes’ in the diagram). However, it does not mean that there exists any
correlation between the biomass and energy. Indeed energy as taken here represents
rate functions or production rates. The relationships between biomass and content
are not fixed. They may differ according to the situations. For example, one gram
of an algae (lesser biomass) may be equal to many grams (more biomass) of a forest
tree leaves as the rate of production of the algae is higher than that of tree leaves.Y-shaped energy flow model-Two channel energy flow model
Following the example of Lindeman, several authors described energy flow modes for
different kinds of ecosystems. Two illustrations are here:
(1) Teal (1957) prepared an energy flow diagram of Root Spring in U.S.A.
(2) H.T. Odum (1957) prepared energy flow model for Silver Springs, Florida, U.S.A.
(3) 30, 810 Kcal/m2 y remained for net production.
In model given by Teal (1957) for Root Springs, most of the energy rich material eaten
by heterotrophs entered the systems as plant debris. On the other hand in the model given
by H.T. Odum (1957) for Silver Spring, most of the heterotroph’s food in food chain was
produced by green with in some systems heterotrophs consume living plants while in others
they feel on dead plant parts (detritus).
(1) In Root Springs, the chain began with dead plant parts.
(2) In Silver Springs the chain began with live plant parts.